1. Field of the Invention
The present invention relates to an optical device which includes an optical thin-film filter, and an optical instrument such as an imaging instrument and an endoscope, which includes this optical device.
2. Description of the Related Art
Various optical devices have hitherto been known as an optical device which includes an optical thin-film filter.
A structure which is proposed as one of the optical devices is a notch filter having 45 degree angle of incidence, which reflects a green color light G from among a red color light R, a green color light G, and a blue color light B, and allows the red color light R and the blue color light B to pass. By regulating a high refractive index material (hereinafter, called as “H”), a low refractive index material (hereinafter, called as “L”), and a ratio of an optical film thickness thereof respectively, an angle dependence with respect to a S polarization is reduced.
Moreover, a structure proposed as another optical device is a dichroic filter having 45 degree angle of incidence of an edge filter type which divides a wavelength band into two. The dichroic filter regulates the refractive index “H”, the refractive index “L”, which form a multilayer film, and the ratio of the optical film thickness thereof respectively. Accordingly, the angular dependence with respect to the S polarized light is reduced. Furthermore, a ripple of a short-wavelength passing region with respect to S polarization is reduced.